TSC2005
www.ti.com
SBAS379–DECEMBER 2006
OVERVIEW (continued)
Measure X-Position
X+
Y+
Touch
X-Position
X-
Y-
Measure Z1-Position
Y+
X+
Touch
Z1-Position
X-
Y-
Y+
X+
Touch
Z2-Position
X-
Y-
Measure Z2-Position
Figure 21. Pressure Measurement
When the touch panel is pressed or touched and the drivers to the panel are turned on, the voltage across the
touch panel will often overshoot and then slowly settle down (decay) to a stable DC value. This effect is a result
of mechanical bouncing caused by vibration of the top layer sheet of the touch panel when the panel is pressed.
This settling time must be accounted for, or else the converted value will be in error. Therefore, a delay must be
introduced between the time the driver for a particular measurement is turned on, and the time a measurement
is made.
In some applications, external capacitors may be required across the touch screen for filtering noise picked up
by the touch screen (for example, noise generated by the LCD panel or back-light circuitry). The value of these
capacitors provides a low-pass filter to reduce the noise, but will cause an additional settling time requirement
when the panel is touched.
The TSC2005 offers several solutions to this problem. A programmable delay time is available that sets the
delay between turning the drivers on and making a conversion. This delay is referred to as the panel voltage
stabilization time, and is used in some of the TSC2005 modes. In other modes, the TSC2005 can be
commanded to turn on the drivers only without performing a conversion. Time can then be allowed before the
command is issued to perform a conversion.
The TSC2005 touch screen interface can measure position (X,Y) and pressure (Z). Determination of these
coordinates is possible under three different modes of the A/D converter:
•
•
•
conversion controlled by the TSC2005 initiated by detection of a touch;
conversion controlled by the TSC2005 initiated by the host responding to the PENIRQ signal; or
conversion completely controlled by the host processor.
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